Processes for converting light hydrocarbon gases, such as natural gas, to heavier synthetic liquid hydrocarbons are generally known in the prior art. Such processes typically involve the "indirect" conversion of gaseous methane to liquid paraffinic hydrocarbon compounds, for example, as disclosed in U.S. Pat. No. 4,833,170. In accordance with the "indirect" conversion process of U.S. Pat. No. 4,833,170, the methane is first converted to a synthesis gas containing hydrogen and carbon monoxide by autothermal reforming, wherein the methane is reacted in a reformer with air in the presence of steam. The synthesis gas is then conveyed to a Fischer-Tropsch reactor which houses a hydrocarbon synthesis catalyst. The synthesis gas is converted to a liquid in the presence of the hydrocarbon synthesis catalyst and the resulting liquid effluent is discharged from the Fischer-Tropsch reactor. The liquid effluent comprises a hydrocarbon phase and an aqueous phase, with the paraffinic hydrocarbon compounds retained in the hydrocarbon phase. The hydrocarbon phase is separated from the aqueous phase upon discharge of the liquid effluent from the Fischer-Tropsch reactor and the synthetic paraffinic hydrocarbon compounds are recovered from the hydrocarbon phase as the liquid hydrocarbon product.
Although an often stated objective of prior art Fischer-Tropsch processes is to reduce the fraction of waxes produced within the mix of liquid paraffinic hydrocarbon compounds, in practice, many prior art Fischer-Tropsch processes, nevertheless, produce a significant fraction of waxes. Waxes are generally defined as relatively heavy paraffinic hydrocarbon compounds having a carbon number in excess of twenty, which are typically in a solid state at ambient temperatures. The presence of waxes in the liquid effluent from the Fischer-Tropsch reactor poses a substantial problem with respect to subsequent handling and transporting of the liquid hydrocarbon product because the waxes tend to coalesce and solidify as the liquid hydrocarbon product cools to ambient temperature. The solid waxes render the hydrocarbon product considerably more difficult to handle and transport, for example, via tanker transport vehicles or product distribution pipelines which are susceptible to fouling and plugging in the presence of solid waxes. One solution to this problem is to maintain the liquid hydrocarbon product at a temperature above the melting point of the waxes at all times. However, this solution is energy-intensive and, therefore, relatively costly. An alternate solution is to perform hydrotreating and mild hydrocracking of the liquid hydrocarbon product after it exits the Fischer-Tropsch reactor to break down the long chain waxes into shorter chain hydrocarbons which remain liquid at ambient temperatures. However, this solution is capital-intensive and likewise relatively costly.
The present invention recognizes a need for a cost-effective solution to the problem of handling and transporting a liquid hydrocarbon product discharged from a Fischer-Tropsch reactor which has a significant fraction of waxes. Accordingly, it is an object of the present invention to provide a process for treating the liquid effluent of a Fischer-Tropsch reactor to facilitate subsequent handling and transportation of the hydrocarbon phase contained in the liquid effluent. More particularly, it is an object of the present invention to provide a treatment process for the liquid effluent of a Fischer-Tropsch reactor, wherein a stable slurry comprising heavier solid hydrocarbon waxes and lighter liquid hydrocarbon compounds is formed from the liquid effluent. It is another object of the present invention to provide a treatment process for the liquid effluent of a Fischer-Tropsch reactor, wherein the resulting slurry is readily transportable via tanker transport vehicles or distribution pipelines at ambient temperatures. It is another object of the present invention to provide a treatment process for the liquid effluent of a Fischer-Tropsch reactor, wherein the slurry is formed from the liquid effluent in a single cost-effective processing step. It is yet another object of the present invention to provide a treatment process for the liquid effluent of a Fischer-Tropsch reactor, wherein the slurry is formed from the liquid effluent by cooling the liquid effluent in a fluidized bed heat exchanger. It is still another object of the present invention to provide a treatment process for the liquid effluent of a Fischer-Tropsch reactor, wherein an aqueous liquid phase is separated from the liquid effluent during formation of the slurry to reduce the amount of water in the slurry. These objects and others are achieved in accordance with the invention described hereafter.